The present invention relates to a digital first order hold circuit for digitally controlling the control voltage provided to a servo and like analog devices. The invention also relates to circuits useful for waveform synthesis using digitally controlled approximation techniques and is especially useful in severe environmental applications and where long duty service makes prolonged use of analog control devices infeasible.
Waveform generators of the prior art include those which have used either analog or digital control techniques. Analog waveform generators are known which use a voltage controlled oscillator to generate an analog waveform. However, voltage controlled oscillators, and other analog components, have been susceptible to voltage drift, extreme ambient temperature sensitivity, and errors which increase with duty cycle and aging. As an example of a waveform generator using a voltage controlled oscillator, see U.S. Pat. No. 4,251,779 which describes a frequency synthesizer apparatus and method useful in ultrasonic imaging applications.
Aside from the difficulties relating to analog synthesizers, digitally controlled waveform synthesizers typically have been complex and accordingly expensive. For example, U.S. Pat. No. 4,326,173 discloses a phase locked loop circuit wherein the synthesized frequency bears a non-linear relationship to a reference frequency. As another example, U.S. Pat. No. 4,626,787 discloses a frequency synthesizer having N identical digital module stages wherein each stage generates one digit of the final digital frequency.
As another example, it has been determined that the phase locked loop circuit is generally not useful for controlling frequencies in a widely varying environment because the phase locked loop lacks the required circuit stability to accomplish precise frequency control of its output with the rapidity dictated by the very short use times appropriate in employment in certain systems. Thus, one of the limitations of the phase locked loop circuit is that the phase locked loop circuit, in and of itself, is not capable of very rapidly changing from one selected frequency to another. Known attempts to resolve the problem of rapid flexibility for phase locked loop circuits, such as that described in frequency synthesizer circuit, U.S. Pat. No. 4,251,779 have required extremely complicated digital processing circuitry. However, as digital processing becomes more complicated, problems of dedication of central processing time, data acquisition and delay, and the probability of generation of data error, become significant design considerations.
In addition to the foregoing limitations, prior art digital waveform synthesizers have lacked the ability to self correct for the impact of analog generated errors, such as voltage drift, or for digital generated errors, such as transient data or erroneous data input.
Accordingly, there exists a general need for a digitally controlled waveform synthesizer having reduced dependence on analog techniques such that voltage drift, temperature sensitivity, and performance during extreme environmental applications can be optimized.
There also exists a need for a digitally controlled waveform synthesizer having a simple design which is useful for high speed data processing applications and which keeps the delay time for acquiring and processing data to a minimum.
There also exists a need for a device having self-correction features to minimize the impact of error generation.
There also exists a need for a digital first order hold circuit for providing a frequency responsive representation of a waveform over an approximation interval.
What is further desired is a simple digitally controlled waveform synthesizer and method for permitting rapid response to changing frequency requirements without involving very complex digital processing circuitry.